The present invention relates to a cation exchange membrane having excellent electrochemical characteristic, cation selective permeability, chemical resistance, heat resistance and mechanical resistance and a process for producing the same. More particularly, it relates to a cation exchange membrane suitable for an electrolysis of an alkali metal halide and a process for producing the same.
Many studies for improving ion selective permeability and durability of a cation exchange membrane have been made. As the result, cation exchange membranes of styrene-divinyl benzene type crosslinked polymer having sulfonic acid groups or modifier thereof have been developed to be commercially available cation exchange membranes having satisfactory selective permeability and durability.
However, the cation exchange membranes have been required to use under severe conditions as found in the treatments of drainage and waste water or the electrolysis of sodium chloride whereby further improvement of the membranes has been required.
The selective permeability has not been always satisfactory.
The conventional cation exchange membranes have suitable permeation inhibiting property to most of anions, however they have disadvantage of low cation transport number in the case of an aqueous solution of an electrolyte forming hydroxyl ions in comparison with the other case.
It has been proposed to use cation exchange membrane of fluorinated resin having sulfonic acid groups as the functional groups in order to overcome the disadvantages.
The typical cation exchange membranes include perfluorosulfonic acid type membranes (commerciallized from E. I. DuPont: Trade name: Nafion Membrane -110, -120, 315 and -390). These membranes have improved durability, however, they do not have satisfactory cation transport number in an aqueous solution of an electrolyte forming hydroxyl ions.
In order to improve the membrane, it has been proposed to provide a method of converting only one surface of the membrane to have --SO.sub.2 NHZ groups (wherein Z represents hydrogen atom, --NH.sub.4, a cation of alkali metal or alkaline earth metal) as disclosed in Japanese Unexamined Patent Publication No. 44360/1973; a method of forming a neutral layer on one surface as disclosed in Japanese Unexamined Patent Publication No. 108182/1975, and a method of increasing EW (weight of resin having one equivalent of exchange groups) on one surface than EW of the inner layer as disclosed in Japanese Unexamined Patent Publication No. 87183/1976.
However, when the conventional membrane made of the fluorinated polymer having sulfonic acid groups is used as a cation exchange membrane, there is no crosslinkage of chemical bond in the membrane, whereby various troubles are found. In order to overcome the disadvantages, it has been proposed to provide a method of forming partially crosslinkage by reacting a diamine or a polyamine with sulfonyl fluoride groups on at least one surface of the membrane to form sulfonamide as disclosed in Japanese Unexamined Patent Publication No. 92339/1975. Certain disadvantages have been overcome by the method, however, the crosslinked structure is not perfect and the current efficiency has not been satisfactorily improved in an economical use.